1. Field of the Invention
The present invention relates to a packet transmission control apparatus and a packet transmission control method which perform transmission control of packets to a plurality of mobile stations.
The present Invention relates particularly to a packet transmission control apparatus and a packet transmission control method which perform transmission control (scheduling) of downlink packets in a mobile communication system.
2. Description of the Related Art
In a downlink of a mobile communication system, one physical channel can be shared among mobile stations which belong to the radio base station. Hereinafter, the physical channel used in such a case is called a “downlink shared channel”.
In this downlink shared channel, the radio base station controls the transmission order of packets to the plurality of mobile stations with which the radio base station communicates, in accordance with an Instantaneous radio quality between the radio base station and each mobile station, so as to improve throughput that the radio base station can provide, in other words, a system capacity.
This control of packet transmission order by the radio base station is called “scheduling”. It is known that, by applying the scheduling to packet transmission, channel capacity increases, or communication quality improves.
Generally, it is considered that the conventional scheduling targets on packets in which requirements for transmission delays are not so strict.
Incidentally, with regard to standardization of the third generation mobile communication system, so-called IMT-2000, there are “3GPP/3GPP2 (Third-Generation Partnership Project/Third-Generation Partnership Project 2)”. Standard specifications have been developed as “W-CDMA system” in the 3GPP, and standard specifications have been developed as “ocdma2000 system” in the 3GPP2.
In the 3GPP, “HSDPA (High Speed Downlink Packet Access)”, which is a high-speed packet transmission system in the downlink direction, has been standardized based upon a prospect that high-speed and high-capacity traffic will increase especially in the downlink due to downloading from databases and websites and the like, as the Internet has rapidly expanded in recent years.
Moreover, in the 3GPP2, “1x-EV DO”, which is a transmission system only for high-speed data in the downlink direction, has been standardized from the same viewpoint as above. In the “1x-EV DO” of the cdma2000 system, “DO” means “Date Only”.
For example, in the HSDPA, a scheme for controlling a modulation scheme and a coding rate of respective radio channels in accordance with radio condition between each mobile station and a radio base station (this scheme is called, for example, AMCS (Adaptive Modulation and Coding Scheme) in the HSPDA), and the scheduling which is operated in a cycle of few milliseconds, are used as a combination. Thus, it is possible to improve throughput for individual mobile stations as well as throughput of the entire system.
“Round Robin Scheduler” is widely known as a scheduling algorithm in a radio base station. The “Round Robin Scheduler” controls the transmission order of packets waiting for transmission, by assigning the downlink shared channel sequentially to mobile stations (for example, mobile stations #1 to #2 to #3 . . . ) which belong to the radio base station.
Moreover, “Proportional Fairness scheduler” and “Max C/I (Maximum C/I) Scheduler” are known as scheduling algorithms in a radio base station. The “Proportional Fairness Scheduler” and the “Max C/I Scheduler” control the transmission order of packets waiting for transmission based upon the radio condition between the radio base station and each mobile station and the average transmission rate of packets to each mobile station.
The “Proportional Fairness Scheduling” is a scheduling algorithm which assigns a transmission queue and also supports fairness amongst the mobile stations, in accordance with instantaneous changes in downlink conditions of the individual mobile stations.
Hereinbelow, The “Proportional Fairness Scheduling” is briefly described with reference to FIG. 1. FIG. 1 is a flowchart showing the operation of the “Proportional Fairness Scheduler” mounted on a radio base station.
In the “Proportional Fairness Scheduling”, a value of an evaluation function of each mobile station which belongs to the radio base station is calculated based upon the measured radio condition between each mobile station and the radio base station and the measured average transmission rate of packets to each mobile station, and thereafter, a transmission queue is assigned to a mobile station maximizing the value of the evaluation function.
As shown in FIG. 1, in step S1001, the radio base station sets initial values as follows:    n=1 (n: a subscript of a mobile station)    Cmax=0 (Cmax: a maximum value of an evaluation function Cn)    nmax=0 (nmax: a subscript of the mobile station maximizing the    value Cn of the evaluation function)
In step S1002, the radio base station measures elements required in calculating the value Cn of the evaluation function, specifically, an instantaneous radio condition Rn between the radio base station and each mobile station #n, and an average transmission rate Rnof packets to each mobile station #n.
In step S1003, the radio base station calculates the value Cn of the evaluation function, by using the values measured in the step S1002, according to the following equation.
  Cn  =      Rn          Rn      _      
In step S1004, the radio base station determines whether the value Cn of the evaluation function calculated in the step S1003 exceeds a maximum value Cmax of the evaluation function.
Here, Cmax=0. Therefore, the determination in the step S1004 is YES, and in step S1005, the radio base station sets the value Cn of the evaluation function calculated in the step S1003 at the maximum value Cmax of the evaluation function, and also sets “1” at “nmax”.
Thereafter, in step S1006, the radio base station increments “n” by “+1”, and determines whether “n” exceeds “N (the number of mobile stations communicating with the radio base station)” in step S1007.
Where “n” does not exceed “N”, the operation repeats the steps from S1002 to S1006, thus obtaining N values of the evaluation function sequentially.
In step S1008, the radio base station selects a mobile station #nmax maximizing the value Cn of the evaluation function, and assigns a transmission queue to the mobile station #nmax.
A radio base station, on which the “Proportional Fairness Scheduler” is mounted, assigns a transmission queue to each mobile station #n while downlink quality (radio condition) is relatively good. Therefore, higher throughput can be expected in comparison with a radio base station with the “Round Robin Scheduler” mounted thereon.
Furthermore, in the “Proportional Fairness Scheduling”, the radio condition between the radio base station and each mobile station is divided by the average transmission rate of packets to each mobile station, thus lowering the value of the evaluation function of a mobile station with high average transmission rate. Thus, the transmission queue can be assigned with high fairness in terms of time, compared to the “MAX C/I Scheduling” as describe later.
Meanwhile, the “MAX C/I Scheduling” is a scheduling algorithm which assigns a transmission queue to a mobile station with the best downlink quality (radio condition) amongst mobile stations which belong to the radio base station.
In other words, in the “MAX C/I Scheduling”, the same processing as that of the “Proportional Fairness Scheduler” is carried out except that the evaluation function Cn in the processing of the “Proportional Fairness Scheduler” is set so that “Cn=Rn”.
In the case of the “MAX C/I Scheduler”, a transmission queue is assigned to a mobile station with good downlink quality at the beginning of a scheduling cycle.
Normally, a transmission rate of respective packets gets higher depending on the quality of a link. Therefore, in the “MAX C/I Scheduling”, a transmission opportunity is provided to a mobile station with the highest transmission rate.
However, in the “MAX C/I Scheduler”, few transmission opportunities are provided to a mobile station with poor average downlink quality, such as a mobile station located remotely from the radio base station. This causes a problem in that throughput obtained at each mobile station is extremely different from one other.
In other words, the “MAX C/I Scheduler” causes a situation where a mobile station located in the vicinity of the radio base station can obtain extremely good throughput, whereas the remaining mobile stations obtain low throughput.
As described so far, in the conventional mobile communication system, the scheduling has been carried out by setting the above-mentioned evaluation function in consideration of the type of service provided and priority related to the type of the service, based upon the “Round Robin Scheduler”, the “Proportional Fairness Scheduler”, or the “MAX C/I Scheduler”.
In the conventional mobile communication system, not only the evaluation function of one scheduling algorithm, such as the evaluation function of the “Proportional Fairness Scheduling” or the evaluation function of the “MAX C/I Scheduling” is selected, but also an evaluation function which is intermediate between the evaluation functions of the “Proportional Fairness Scheduling” and the “MAX C/I Scheduling” is sometimes selected.
For example, in the conventional mobile communication system, radio condition Rn between the radio base station and each mobile station #n, and an average transmission rate Rnof packets to each mobile station #n are used to set an evaluation function according to
      Cn    =                  Rn        α                              Rn          _                β              ,where α and β are adjusted within a range of 0≦α,β≦1. Thus, it is possible to employ a scheduling method which, for example, controls a degree of fairness provided by the denominator and an effect of user diversity provided by the numerator.
Furthermore, in the conventional communication system, another index (for example, a buffering time of a packet waiting for transmission) may be added to the evaluation function to carry out the scheduling.
Meanwhile, the high-speed packet transmission system like the “HSDPA” and the “1x-EV DO” is generally a communication system of the Best Effort type. This type of transmission system provides packet transmission with a high transmission rate where the number of mobile stations for assignment is small, and provides packet transmission with a low transmission rate where the number of mobile stations for assignment is large.
However, in this high-speed packet transport system, it has also been considered that a service like a streaming service and a VoIP service is to be provided. In such a service, a predetermined requirement for transmission delay should be satisfied.
In other words, in the above high-speed packet transmission system, it is required to set a guaranteed transmission rate to guarantee a minimum transmission rate of packets for a predetermined type of service.
In such a case, the mobile communication system is required to execute its control to guarantee a minimum transmission rate of 64 kbps of packets to an arbitrary mobile station using, for example, a streaming service.
However, in the conventional scheduling algorithms (the “Round Robin Scheduling”, the “MAX C/I Scheduling”, and the “Proportional Fairness Scheduling”, there is not a function which provides a specific type of service or a specific mobile station with a guaranteed transmission rate. Therefore, there has been a problem in that QoS (Quality of Service) cannot be provided to a streaming service, a VoIP service and the like.